Typically, a dump truck that conveys minerals mined from mines, etc. or earth and sand, etc. as a load includes: a frame; front wheels rotatably provided at a front part of the frame; rear wheels rotatably provided at a rear part of the frame; a vessel that is provided to be liftable from the frame and is to be loaded with the load; and left and right pairs of suspension cylinders that intervene between the frame and the front wheels and rear wheels and hold the vehicle body while absorbing vibrations from the road surface during traveling and alleviating shocks to the vehicle body.
The dump truck includes the vessel that is provided to be liftable on the frame and is to be loaded with a load, such as earth and sand and gravel, a hinge pin provided at the rear part of the frame, and hoist cylinders that are arranged on the frame more frontward than the hinge pin and couples the frame and the vessel to each other. The dump truck can dump the load from the vessel by extending the hoist cylinders to push the vessel upward and raise this vessel.
The vehicle weights of the dump truck having such a configuration in cases with and without the load are different by two-fold or more. Consequently, use of suspension cylinders having linear spring characteristics largely changes the vehicle height in the cases with and without the load. Accordingly, the suspension cylinders of the dump truck are required to have nonlinear spring characteristics that the harder, the stiffer the cylinders become. Furthermore, since the own weight of the dump truck and the carrying load of the vessel are significantly large, the holding force characteristics capable of holding the weight and load are also required.
According to the above reasons, suspension cylinders in which gas and oil are seamlessly enclosed are included in the dump truck in order to achieve the nonlinear spring characteristics and exert a large holding force. The suspension cylinder having such a configuration has a double spring structure where gas and oil are compressed by receiving an external force. As one of conventional arts of spring structures of such a type, a fluid pressure type active suspension has been known (e.g., see Patent Literature 1).
When the dump truck accelerates and decelerates and goes over obstacles, the suspension cylinders are extended and retracted. There is a problem in that the extension and retraction cause the movement of the vehicle body oscillating forward and rearward, that is, a pitching movement, which changes the attitude of the vehicle body during traveling. As one conventional art to solve the problem, a vehicle body attitude control device has been proposed that prevents the vehicle body from rising during curving travel by distributing the downforce, which is in proportion to the lateral acceleration, to oil hydraulic suspension mechanisms for front and rear axle wheels and by applying the distributed forces (e.g., see Patent Literature 2).